Computing, communications, switching and interconnection are technical fields which have deemonstrated both applicability and need for optics and optical devices. In these technical fields, one class of device which is needed is an optical logic device. For the optical logic device, data or information carrying signals incident on the device control the state of the device in such a way that some combinatorial (Boolean) or memory (latch) function or some combination or combinatorial (Boolean) and/or memory (latch) functions is performed on the incident signals.
Nonlinear Fabry-Perot etalons have been suggested as all-optical devices which can provide optical logic functions. See S. D. Smith, Applied Optics, Vol. 25, No. 10, pp. 1150-64 (1986) and H. S. Hinton, IEEE Journal on Selected Areas in Communications, Vol. 6, No. 7, pp. 1209-26 (1988). One drawback to the use of nonlinear Fabry-Perot etalons in high speed operation is that incident controlling signals such as clock and data signals must be separated in wavelength so that one wavelength corresponds to an absorption peak of the nonlinear material in the etalon. Such a limitation is necessary to permit switching or tuning the nonlinear Fabry-Perot etalon between transmissive and reflective states. As a result of this method of operation, the input wavelength is different from the output wavelength thereby excluding the possibility of cascading these devices one after the other. While required wavelength differences pose significant limitations, it cannot be avoided that other limitations arise because temperature variations cause the etalon to undergo changes with respect to location of resonance peaks for the cavity. In turn, the etalon may or may not be responsive to input optical signals. Moreover, intensity variations of the incident signals can cause the nonlinear Fabry-Perot etalon to change state in a haphazard manner or not at all.
Self electrooptic effect devices have also been shown to be suitable for operating as sequential memory elements realized as S-R flipflops. See U.S. Pat. Nos. 4,754,132 and 4,751,378. These bistable memory elements are affected by past inputs as well as present inputs. Logical interconnection of these memory elements permits realization of shift register circuits such as a photonic ring counter. See Proceedings of the Conference on Lasers and Electrooptics, paper TUE4 (1988). However, the utility of such photonic memory devices is limited in computing, communications, and switching applications without the existence of photonic combinatorial logic elements such as AND, OR, Exclusive-OR gates and the like.
Recently, symmetric self electrooptic devices have been utilized in a combinatorial logic gate for producing the NOR function. See Proceedings of the Conference on Lasers and Electrooptics, paper TUE4 (1988). In this paper, it was stated that OR, NAND and AND functions were demonstrated. Proper operation of the devices described requires that each optical data signal and its logical complement be supplied to the logic gate. As a result, additional hardware such as a beam splitter and an optical inverter are required for each optical data signal to obtain a complementary data signal, if one is not readily available. Even though AND and OR logic gates are shown or described in the reference cited above, it should be noted that the reference nowhere suggests the realization of and exclusive-OR gate. Exclusive-OR gates are important because it is not possible to realize photonic encoders or scramblers or their inverses without Exclusive-OR gates in the feedback path and at other interconnection points of a photonic shift register.